It is well known that in the development of the ISDN network, besides the usual telephone speech and data transmission services, way to the home subscribers, among which transmission of documents by Group 4 facsimile equipment and reception and transmission of photo images at CCIR standard resolution, should be offered.
The Group 4 facsimile is a real time transmission method of texts and images, sent onto the ISDN network with a particular coding and a 64 kbit/s rate, in which the whole process, from the composition moment (acquisition through a scanner, acquisition from the memory of a processor, etc.) up to the reception and printing, is of digital type. Noise immunity is very high and resolution is of 400 pixels per inch, higher that than of the analog facsimile presently used.
There is also the possibility of transmitting photo images, locally obtained with a TV camera, residing in the memory of a processor or received from a distant data bank. Their resolution is that as from studio, foreseen in CCIR 656 recommendation, so that they can be displayed with suitable means in order to obtain good quality images with the same transmission resolution. This type of transmission can be used in a variety of different fields, e.g. it can meet the requirements of estate agents, tour operators, and the like, who could consult centralized data bases regrouping the offers available on the market at that moment. It could resolve the problem of the creation of a general archive of the works of art, facilitating the transmission of the images of paintings, monuments and others to the culture ministry and hence their viewing by the public.
To be able to access in a capillary way all these services, offered within an ISDN network, the subscribers must be equipped with suitable terminals capable of allowing the transmission, the reception, the display and possibly the printing of the exchanged documents. The costs of such terminals can be reduced by the use of the computer which is already widespread in private houses. With the addition of a suitable circuit and a suitable program, it can become the first and more versatile terminal of the ISDN network.
The circuit, which can be housed on one or more printed circuit boards, will permit the computer to carry out complex processing operations of the video signal in real time. It must comprise a video signal memory to be used for image acquisition from a TV camera or for display of images residing in the personal computer itself. Besides it must be equipped with interface circuits for the digital ISDN network. A suitable circuit allowing the computer monitor, operating with its particular TV standard, to display the images with the characteristics specified in CCIR 656 recommendation, also must be provided. Thus resorting to a specific monitor only for displaying the images with the specified characteristics is avoided.
The control program can be permanently stored under firmware format. To date there does not exist on the market a video control circuit capable of carrying out all the operations carried out by the circuit provided by the invention. There exists on the contrary only a series of devices for carrying out some partial functions. They are generally dedicated to the processing of images and particularly to the graphics, and are used in the video control units installed in personal computers and workstations dedicated to CAD processes.
E.g. there are programmable integrated circuits, named "Colour video controller", such as those identified by IMS G300B and described on page 87 and ff. of the catalogue 1990 of INMOS, which implement the real-time control functions of a graphic system and of its display means, e.g. a cathode ray tube (CRT), a laser printer or a color plotter. They can be interfaced with the bus of each standard processor, wherefrom they receive the programs necessary to specify timings and data display characteristics on the screen of the CRT or on a printed sheet. They can also be interfaced with the source of data to be represented, using in both cases dual-port memories which allow speed up of the procedures to the maximum. These controllers contain a video timing generator, completely programmable by the external microprocessor, which generates video synchronism, addresses, data timing and all the display characteristics required, such as: image dimension, vertical and horizontal scanning frequency, presence of interlacing, etc. They contain in addition three totally-programmable memories, the so called "color look-up tables", allowing the representation of output data with a certain number of color chosen out of a larger set. The three RGB flows outgoing from the table are converted into analog form to be directly sent to the video screen.
Moreover, there is another type of integrated circuit, such as those identified by IMS G178, described on page 63 and ff. of the above catalogue 1990 of INMOS, which implement the already-described "Colour look-up table" function, i.e. the storing of the color values to be associated to different screen points. They comprise three biport memories, generally of small capacity, each relative to one of the primary colours processed (RGB=red, green, blue). Each memory is loaded and the contents is updated by the microprocessor controlling the system. The reading of the memories is carried out by using instantaneous values of RGB data flow to be displayed as addresses, each of them corresponding to a suitable value which is presented at the output gate. The three values obtained thus, converted into analog form, form the term determining the color of a pixel of the screen.
As mentioned, these integrated circuits are particularly oriented towards graphic applications in personal computer and workstations, by implementing the typical specifications of graphic cards, such as VGA, EGA, etc. Besides they are apt to process the signals in the format of the fundamental RGB colors, while CCIR standards require that the photographic images be processed in the format of luminance and chrominance components.